The present invention relates generally to the field of furnaces and boilers, and in particular to new and useful apparatus and method of efficiently mixing gas streams containing particles, with each other.
The present invention is generally drawn to devices for distributing and mixing particle or injected gas laden air in ducts and more particularly to such devices as used in the ducts of power generating stations which may contain ammonia for NOx reduction apparatuses.
It is known to use air foils for distributing and mixing air streams in secondary air supply ducts and selective catalyst reduction (SCR) system flues. The usual arrangement comprises a plurality of whole foils in the center of the flue and half foils at the walls of the flue. Another example of prior art air foil uses an air foil configuration for distributing and mixing economizer bypass flue gas used in the Kansas City Power & Light, Hawthorn Station in their SCR flue system. This system uses a basic system of air foils but has gas-flow ordering plates added. Contour lines in an airflow diagram of such a device show how the airfoils and plates act in the air stream to enhance mixing of the gases in the duct. See Published US Patent Application 2006/0266267 to Albrecht et al, herein incorporated by reference.
In addition, air foils have been used extensively for flow measurement and control. It is also known to use Diamond shaped flow devices for flow control with low pressure drop. For example, many commercially available dampers contain diamond shaped blades. Such devices achieve good flow control with minimal pressure drop.
Disadvantage of the above described prior art arrangements are added pressure loss, potential degradation mixing of ammonia when added, and the requirement for a larger flue to accommodate the system components. Ammonia injection grids (AIG) with zone control are known and have been installed to distribute a prescribed rate of ammonia for NOx reducing SCR systems. Static mixers are commercially available in several forms and have been proposed to reduce thermal and/or flue gas species gradients by adding turbulent mixing in SCR flue systems. Koch and Chemineer are manufacturers that produce some such commercially available static mixers. Design requirements for secondary flues and SCR systems include the specification of flow distribution and thermal gradients downstream of the mixing devices. The objectives are to achieve flow uniformly and minimize thermal gradients. For example, in an SCR system mixing and flow uniformity at the ammonia injection grid should be sufficient such that catalyst performance and life is maintained. To accomplish these goals, devices such as those of the prior art have been utilized. While it is also desirable to minimize the unrecoverable pressure loss to the system, space restrictions limit the installation of an air foil for gas mixing and a separate AIG for ammonia distribution in an SCR system. Thus a uniform distribution system for such applications was needed which would also minimize the pressure loss therein.
Published US Patent Application 2006/0266267 to Albrecht et al., mentioned above, discloses a flow enhancing arrangement for ducts such as rectangular flue ducts wherein a series of tear shaped foils are spaced from each other and mounted in the duct extending from top to bottom thereof and where a series of diamond shaped vanes also extending from the top to the bottom of the duct are spaced and mounted between tear shaped foils to provide a more uniform flow distribution and to lower the pressure thereby. A series of baffles extending from both the tear shaped foils and the diamond shaped vanes may also be used.
U.S. Pat. No. 6,887,435 to Albrecht et al., herein incorporated by reference, discloses an integrated air foil and ammonia injection grid provides a plurality of air foils across a flue conveying flue gas. Each air foil has a leading curved edge and a tapered, pointed, trailing end. At least one injection pipe is positioned inside each air foil, and has at least one nozzle for injecting ammonia into the flue gas flowing across the air foils. Preferably, plural injection tubes are provided and positioned one behind the other in each air foil, and each injection tube in a given air foil has a length different than a length of the other injection tubes in the same air foil. A longest injection tube in a given air foil is located furthest downstream and proximate the tapered trailing edge and a shortest injection tube in the same air foil is located furthest upstream, remaining injection tubes in the same air foil being progressively shorter the further upstream any injection tube is located. Apertures may be provided on opposed lateral sides of the air foils for introducing a gas flow into the flue gas passing across the air foils. Ammonia flow to each injection pipe may be individually controlled.
U.S. Pat. No. 4,980,099 to Myers et al. discloses an apparatus for spraying an atomized mixture into a gas stream comprises a stream line airfoil member having a large radius leading edge and a small radius trailing edge. A nozzle assembly pierces the trailing edge of the airfoil member and is concentrically surrounded by a nacelle which directs shielding gas from the interior of the airfoil member around the nozzle assembly. Flowable medium to be atomized and atomizing gas for atomizing the medium are supplied in concentric conduits to the nozzle. A plurality of nozzles each surrounded by a nacelle are spaced along the trailing edge of the airfoil member.
Air foils for distributing and mixing gas streams have been used in secondary air supply ducts and selective catalyst reduction (SCR) system flues. The arrangement consists of a plurality of whole foils in the center of the flue and/or half foils at the wall of the flue as used for the Eastman Kodak facility identified above.
Another example of an air foil configuration for distributing and mixing economizer bypass flue gas was used in the Kansas City Power & Light, Hawthorn Station SCR flue system. In addition, air foils have been used extensively for flow measurement and control. Ammonia injection grids (AIG) with zone control have been installed to distribute a prescribed rate of ammonia for NOx reducing SCR systems. Static mixers are commercially available in several forms and have been proposed to reduce thermal and/or flue gas species gradients by adding turbulent mixing in SCR flue systems. Koch and Chemineer produce some examples of commercially available static mixers.
Diamond shaped flow devices have been used for flow control with low pressure drop. For example, many commercially available dampers contain diamond shaped blades. Such devices achieve good flow control with minimal pressure drop.
Design requirements for secondary flues and SCR systems include the specification of flow distribution and thermal gradients downstream of the mixing devices. The objectives are to achieve flow uniformity and minimize thermal gradients. In addition, space restrictions limit the installation of an air foil for gas mixing and a separate AIG for ammonia distribution in an SCR system.
Alternatives are to use air foils to distribute the flue gas within the flue and to include plates or baffles to promote flow mixing in the flue/duct. The disadvantage of such an arrangement is added pressure loss, potential degradation mixing, and a larger flue to accommodate the system components.
A need remains for an effective and simple apparatus for mixing of gas streams, in particular streams of different temperatures and/or compositions, and that contain particles such as ash.